1. Field of the Invention
The present invention relates to a surface acoustic wave device and a communication system using it wherein a plurality of surface acoustic waves are propagated on a substrate and a signal produced with the interaction of these surface acoustic waves is picked up by the use of the physical non-linear effect of the substrate.
2. Related Background Art
The surface acoustic wave device has been increasingly important in recent years,.as a key device in making the spread spectrum communication. Also, it has been noted in many applications as the real-time signal processing device and studied actively.
FIG. 1 is a schematic plan view showing an example of such a conventional surface acoustic wave device.
In the same figure, there is provided on a piezoelectric substrate 1 pair of input interdigital transducers 2 and a central electrode 3 therebetween. The transducers 2 are electrodes for exciting surface acoustic wave signals, while the central electrode 3 is an electrode for propagating the surface acoustic wave signals in opposite directions to each other and for picking up an output signal.
If a signal F(t)exp(j.omega.t) is applied to one of the transducers 2, and a signal G(t)exp(j.omega.t) to the other, two surface acoustic waves in opposite directions to each other EQU F(t-x/v)exp[j.omega.(t-x/v)]
and EQU G(t-(L-x)/v)exp[j.omega.(t-(L-x)/v)]
will propagate on a surface of the piezoelectric substrate 1. Where v is the velocity of surface acoustic wave and L is the length of central electrode 3.
On this propagation path, a product component of above surface acoustic waves is yielded due to the non-linear effect, integrated over a range of the central electrode 3, and then picked up. This output signal H(t) can be represented by the following expression. EQU H(t)=.alpha..multidot.exp(j2.omega.t).intg..sub.o.sup.L F(t-x/v)G(t-(L-x)/v)dx
Where .alpha. is a proportional constant.
Thus, a convolution signal of two signals F(t) and G(t) can be obtained from the central electrode 3.
However, as such a constitution has generally a lower efficiency, a surface acoustic wave device as shown in FIG. 2 has been proposed by Nakagawa et al. in "Electronic communications society journal" 1986/2, Vol. j69-C, No. 2, pp190-198. Note that the axis of coordinate as shown in FIG. 2 was appended for convenience, but not meaning the crystal axis of substrate.
In FIG. 2, 11 is a piezoelectric substrate, and 12, 13 are two input interdigital transducers for excitation of surface acoustic waves formed on a surface of the substrate 1, opposed to each other and spaced by an appropriate distance in the x direction, 14-1, 14-2, . . . , 14-n are waveguides formed on the surface of substrate 11 extending in parallel to each other in the x direction between the transducers 12, 13, and 15 is an output interdigital transducer formed on the surface of the substrate 11, spaced by an appropriate distance in the y direction from the above-mentioned waveguides.
In this surface acoustic wave device, if an electric signal with an angular frequency .omega. is input to the transducers 12, 13 for excitation of surface acoustic waves, the surface acoustic waves of that frequency will be excited, and propagate on the waveguides 14-1, 14-2, . . . , 14-n in the x direction but in opposite directions to each other, in which a surface acoustic wave with an angular frequency 2.omega. propagating in the y direction will be produced on the waveguides due to the parametric mixing phenomenon. This surface acoustic wave arrives at the output transducer 15 in which a convolution electric signal for two input signals as above indicated can be obtained.
However, with the surface acoustic wave device as shown in FIG. 2, there was a problem that the reflected waves might be produced at side edges (end faces) of the waveguide for surface acoustic wave, interfering with the surface acoustic wave indicating the signal passing toward the output transducer. And with the conventional surface acoustic wave device as above described, each divided waveguide is formed at a pitch equal to a whole multiple of the wavelength of surface acoustic waves produced by these waveguides, so that surface acoustic waves reflected at an end face on the same side in the y axis direction of each waveguide are intensified with each other, thereby increasing the reflectance on all of the waveguides. Thus, there was a problem of having a bad effect on the output signal of the surface acoustic device, such that the waveguides and the output transducer may constitute a resonator, producing the distortion or ripples in the frequency characteristics of the convolution signal, for example.